1. Field of the Invention
The present invention relates to a radiation-image conversion panel for use in the radiation-image conversion techniques utilizing stimulable phosphors, and a process for producing such a radiation-image conversion panel.
2. Description of the Related Art
The computed radiography (CR) is known as a technique for recording and reproducing a radiographic image. In the computed radiography (CR), a system constituted by a radiographic-image record device, a radiographic-image reading device, and the like is used, and the stimulable (photostimulable) phosphor is used in the system. When the stimulable phosphor is irradiated with radiation such as X-rays, a portion of the energy of the radiation is accumulated in the stimulable phosphor. Thereafter, when the stimulable phosphor is irradiated with excitation light such as visible light, the stimulable phosphor emits stimulated luminescence the amount of which corresponds to the accumulated energy of the radiation. The radiographic-image record device temporarily records a radiographic image of an object such as a human body as a latent image in a stimulable-phosphor layer, and the radiographic-image record device irradiates the stimulable-phosphor layer with excitation light such as laser light so as to emit stimulated luminescence, and photoelectrically detects the stimulated luminescence so as to acquire an image signal representing the radiographic image of the object.
The radiation-image conversion panel is known as a recording medium used in the above system. The radiation-image conversion panel is formed, for example, by laminating a rigid substrate, a stimulable-phosphor layer, and a protective layer. It is possible to remove radiation energy remaining in the stimulable-phosphor layer by irradiating the stimulable-phosphor layer with erasing light after the radiographic image is read from the stimulable-phosphor layer. Therefore, it is possible to repeatedly record and read radiographic images in each radiation-image conversion panel. In order to suppress chemical deterioration of and physical impact on each stimulable-phosphor layer (which are caused by the repeated use of each stimulable-phosphor layer), normally a protective layer is formed over the stimulable-phosphor layer (i.e., on a surface of the stimulable-phosphor layer on the side opposite to the substrate).
Generally, the protective layer of the radiation-image conversion panel is made of a resin which is transparent to the excitation light and the emitted light. However, when the thickness of the protective layer is small, unevenness (such as interference fringes) or imperfection is likely to occur in the radiographic image. In order to solve this problem, it has been proposed to make the protective layer have a certain haze value, or arrange a antireflection layer over the protective layer. Japanese Unexamined Patent Publication No. 8(1996)-201598 discloses a radiation-image conversion panel having an antireflection layer made of an organic solvent-soluble material and arranged on a transparent protective layer, where the surface of the antireflection layer is made of a fluorine resin, and the surface reflectances of the antireflection layer at incident angles of 0 to 60 degrees of the light having a wavelength in the ranges of 30 to 500 and 600 to 900 nm do not exceed 10%.
Japanese Unexamined Patent Publication No. 2002-098799 discloses a radiation-image conversion panel having a protective layer which is formed of a material containing aluminum oxide or silicon oxide by evaporation, and has a gaseous barrier property exhibiting the water-vapor transmission rate of 2 g/m2/24 h or lower and the oxygen transmission rate of 2 cc/m2/24 h or lower. Since the protective layer in this radiation-image conversion panel is made by evaporation of an inorganic material having a gaseous barrier property, the protective layer can be formed as a thin film. In addition, since the thin film is less likely to produce image blur caused by light scattering, the quality of the initial image obtained by use of the radiation-image conversion panel is improved.
Further, U.S. Patent Laid-Open No. 20030146395 discloses a radiation-image conversion panel having a protective layer and transparent inorganic layers and organic layers, where the transparent inorganic layers and organic layers are alternately formed, the number of the transparent inorganic and the organic layers is four or more, and the protective layer has a thickness of 50 micrometers or smaller, an air transmission rate of 0.5 cc/m2/24 h or lower, and a water-vapor transmission rate of 0.5 g/m2/24 h or lower. The transparent inorganic layers are made of metal oxides, metal nitrides, or metal oxynitrides, and formed by vacuum deposition. The organic layers are formed by application or vacuum deposition. Therefore, the moisture resistance of the radiation-image conversion panel can be enhanced. In addition, since it is unnecessary to bond the layers to each other with an adhesive, it is possible to prevent occurrence of image blur caused by the bonding, and form the transparent protective layer so that the total thickness of the transparent protective layer is reduced. Thus, high quality images can be obtained.
As mentioned above, the protective layer disclosed in Japanese Unexamined Patent Publication No. 2002-098799 is formed by evaporation, and the transparent inorganic layers and/or the organic layers in the protective layer disclosed in U.S. Patent Laid-Open No. 20030146395 are formed by vacuum deposition. However, when an inorganic layer is formed on a support (e.g., a PET film) for the protective layer or an organic layer by vacuum deposition, distortion or wrinkles are likely to occur in the protective layer, and therefore the quality of the images obtained by use of the radiation-image conversion panel deteriorates.
In order to suppress the occurrence of distortion and wrinkles during formation of the protective layer, it is necessary to increase the thickness of the support or the organic layers on which the inorganic layers are to be formed. However, when the thickness of the support or the organic layers is increased, optical scattering causes image blur, and therefore it becomes difficult to obtain satisfactory images. In addition, if, as an alternative to the use of the support or the organic layers, an inorganic layer is directly formed on a stimulable-phosphor layer by vacuum deposition, it is difficult to form a uniform inorganic layer by evaporation in the case where the stimulable-phosphor layer has a gap structure constituted by columnar crystals as in the radiation-image conversion panel disclosed in Japanese Unexamined Patent Publication No. 2002-098799.